Non-thermal plasma-catalytic reforming of tar over Ni-based catalysts

The production of tar as a by-product during biomass gasification to syngas (H₂/CO) will clog the gasification equipments and cause environmental pollution, hindering the development of gasification technology. The combination of non-thermal plasma and nickel-based catalysts can take advantage of the low-temperature reaction of plasma and the high target product selectivity of the catalyst to transform tar into syngas. However, nickel particles are easy to agglomerate at high temperatures, and the catalyst is easily deactivated by carbon deposition. Therefore, designing and modifying catalysts to improve their anti-carbon deposition properties has become the key to solve the catalyst deactivation problem. This paper reviewed the deactivation principle of nickel-based catalysts, the design and modification of catalysts to improve their anti-carbon properties, the types of plasma reactors and mechanisms of non-thermal plasma reforming of tar, and the synergistic effect of non-thermal plasma and catalysts in the non-thermal plasma catalytic reforming tar systems in recent years. The outlooks of plasma-catalytic reforming of tar is also prospected. The design and modification of nickel-based catalyst mainly depend on three factors: carrier, auxiliary agent and preparation method. The specific surface area, pore volume and acidic site of the support are critical factors affecting the Ni dispersion, the interaction between metal and support, the tar cracking reaction rate and the carbon deposition formation rate. On the one hand, metal auxiliaries can form bimetals with active metals, and form strong interaction between metals and carriers; On the other hand, alkali metals can interact with Brønsted strong acid sites to form weak Lewis acid sites through their own alkalinity, improving carbon resistance via reducing acidity of catalyst and accelerating oxygen transfer. Among the preparation methods, coprecipitation method and sol-gel method are more conducive to the improvement of metal dispersion, the reduction of catalyst particle size, and the enhancement of carbon deposition resistance of catalyst. In the reactor of non-thermal plasma-catalytic reforming of tar, dielectric barrier discharge reactor is preferred in tar reformation due to the advantages of small production of carbon deposition and uniform distribution of electrons and moderate energy consumption. The main active particles in the non-thermal plasma catalytic system are e^*, ·O, ·OH, ·H and N^*, in which high-energy electrons can dissociate the tar molecules through collision, and the pyrolysis products are oxidized by the reactive oxygen species ionized by water vapor and on the catalyst surface, and new products are generated along with the mutual combination of free radicals. There is a synergistic effect between the non-thermal plasma and the catalyst. The presence of the catalyst enhances the electric field of the plasma, and micro-discharge is formed in the pore structure of the catalyst, which promotes the reaction. The presence of plasma activated the active species on the catalyst surface and added a new reaction path for tar reforming. Future research should explore which one of the non-thermal plasma and catalysis systems plays a leading role, and optimize the leading system. This paper provides a reference for the design and development of nickel-based catalysts in non-thermal plasma catalytic reforming tar system.